MOHOROVIčIć, ANDRIJA

Mohorovičić’s father was a shipwright; his mother died shortly after his birth. A brilliant pupil at the grammar school in Rijeka, he entered the University of Prague in 1875, graduating in mathematics and physics. One of his teachers at Prague was Ernst Mach. There followed a period as a secondary school teacher, and in 1882 he was appointed to the Royal Nautical School in Bakar (Buccari), near Rijeka, where he taught, among other things, meteorology and oceanography. His interests turned strongly toward meteorology and in 1887 he founded the Meteorological Station of Bakar. In 1891 he was appointed professor at the Main Technical School in Zagreb, where in 1892 he became director of the meteorological observatory. In 1897 he received his doctorate from the University of Zagreb, becoming an unsalaried lecturer there the same year and reader in 1910.

Most meteorological centers in Croatia had been set up under control from Budapest. Mohorovičić campaigned actively to remove this control and in 1900 he succeeded in having the Zagreb observatory established as meteorological center for all Croatia and Slavonia, completely independent of Budapest, as the Royal Regional Center for Meteorology and Geodynamics. Soon after the turn of the century, seismology became his dominant interest and the field in which he became famous. He retired in 1921 when the reorganized Royal Regional Center was renamed the Geophysical Institute. Although troubled by weak eyesight since 1916, he continued in active seismological research until 1926.

Mohorovičić’s fame rests nearly entirely on the results of his very thorough investigation of the destructive earthquake of 8 October 1909, which occurred about thirty miles south of Zagreb, in the Kulpa valley of Croatia. Previously the velocities of Pand S (primary and secondary) seismic waves had been treated as varying continuously with the depth, apart from minor discontinuities connected with geographic features such as sedimentary deposits, mountain ranges, and ocean floors. In his study of records of the Kulpa valley earthquake, however, Mohorovičić detected the presence of two distinct pairs of P and S phases recorded by seismographs at distances between 125 and 450 miles from the epicenter of the earthquake. His careful analysis showed that one of the pairs was associated with markedly slower speeds of wave travel. Mohorovičić correctly interpreted the observations as showing that the focus of the earthquake was inside a distinct outer layer of the earth; that the slower P and S waves had traveled directly to observing stations through this layer; and that the paths of the faster waves had been mostly below the layer, having been refracted when passing down from the focus through the separating boundary and later refracted upward through this boundary to the surface. He calculated the thickness of the layer as 30–35 miles. The work of other seismologists later confirmed this discovery, though reducing his estimate of the layer thickness, and showed that the layer was worldwide.

This outer layer of the earth is now conventionally called the crust, and the region between the crust and the central core (at a depth of 1,800 miles) is known as the mantle (originally the shell). The boundary separating the crust and the mantle is now called the Mohorovičić discontinuity (sometimes the M discontinuity). Its depth below the earth’s surface is nowknown to be about 20 miles in continental shield areas, somewhat greater under large mountain ranges, but only 3–6 miles below ocean floors. Across the Mohorovičić discontinuity, the seismic P velocity changes fairly rapidly from about 4 to 5 miles per second and the S velocity from about 2½ to 3 miles per second. The Mohorovičić discontinuity was the second major boundary to be discovered below the earth’s surface by seismological means; in 1906 R. D. Oldham had established the existence of the central core. The depth of the boundary separating mantle and core was accurately determined by Beno Gutenberg in 1914.

One outcome of Mohorovičić’s work on the 1909 earthquake was a classification of earthquakes for investigative purposes into near and distant earthquakes. Near earthquakes are well recorded within about 600 miles of the epicenter and within this range show low-velocity phases of the type that Mohorovičić had found. In distant earthquakes the dominant interest is on records taken beyond this distance.

The name of Mohorovičić caught the public fancy in 1957 when the International Association of Seismology and Physics of the Earth’s Interior sponsored a proposal, the Mohole project, to drill through the earth’s crust to just below the Mohorovičić discontinuity. In the hands of journalists the discontinuity soon came to be referred to as the Moho.

As a meteorologist Mohorovičić was noted for his great organizational ability, his insistence on high standards of precision wherever he had responsibilities, and his success in circumventing bureaucracy. His research papers dealt with such subjects as cloud movements, the variation of atmospheric temperalure with height, rainfall in Zagreb, and a tornado. Mohorovičić was also noted for his deep appreciation of what was needed to produce an up-to-date seismological observatory. In 1901, soon after the Royal Regional Center had been set up, a strong earthquake was felt in Zagreb, as a consequence of which he and a colleague were able to secure for the center a reliable seismograph constructed by the noted Italian seismologist G. Agamennone. With the addition of further equipment, including a Wiechert seismograph and equipment for ensuring precise timing, the center had become by 1908 one of the leading seismological observatories in central Europe. The thoroughness of this preparation was a major factor in the success of his study of the 1909 Kulpa valley earthquake.

Mohorovičić contributed research papers on a variety of other topics in seismology. He evolved a method of determining earthquake epicenters and constructed curves giving the travel times of seismic waves over distances of up to 10,000 miles from the source. He also carried out macroseismic studies—the investigation of the salient features of an earthquake from reports of observations of surface effects (geological effects, effects on buildings, bridges, among others) taken over a wide area surrounding the epicenter, and he investigated the subject of constructing earthquake-proof buildings. His ideas on improving the construction of seismographs were unrealized due to insufficient financial support.

Mohorovičić was most punctilious, tenacious, and meticulous in all his scientific work, and was known for always talking to the point. He was keen and enthusiastic in his undertakings and intolerant of slipshod work. Even after his eyesight began to fail, he worked far into the night reading specialized papers on seismology. An extremely good-humored man, he was idolized by his colleagues. His success was partly due to his unusual linguistic ability. At the age of fifteen, he spoke Croatian, Italian, English, and French, and later spoke German, Czech, Latin, and classical Greek as well. His son Stjepan is also a distinguished seismologist. Mohorovičić was elected to the Yugoslav Academy of Sciences in 1898 and was secretary of the mathematics and science section from 1918 to 1922.

BIBLIOGRAPHY

I. Original Works. Mohorovičić’s best-known publication is “Das Beben vom 8.X.1909,” in Jahrbuch des meteorologischen Observatoriums in Zagreb for 1909, pt. 4, par. 1 (1910), 1–67. The essential content of this paper was published in French by E. Rothé, “Sur la propagation des ondes sismiques au voisinage de l’épicentre. Préliminaires continues et trajets à réfraction,” in Publications da Bureau central séismologique international, ser. A, Travaux scientifiques, fasc. 1 (1924), 17–59. Mohorovičić published a total of 21 papers on meteorology and seismology from 1888 to 1926, mostly in journals published in Zagreb; a full list is held by the University of Zagreb.

II. Secondary Literature. The proceedings of a symposium held in Zagreb in March 1968 in honor of Mohorovičić include an article by his student and collaborator Josip Mohoroviči, “Andrija Mohorovičić—sein Leben and Wirken.”

K. E. Bullen

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Mohorovicic, Andrija (1857-1936)

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Mohorovicic, Andrija (1857-1936)

Croatian seismologist and meteorologist

Croatian seismologist and meteorologist, Andrija Mohorovicic was the first one to suggest the existence of a boundary surface separating the crust of the earth from the underlying mantle. This layer, which is 5 mi (8 km) deep under the oceans and about 20 mi (32 km) deep under the continents in average, was later named the Mohorovicic discontinuity . In 1970, a large crater on the far side of the Moon was also named in Mohorovicic's honor.

Mohorovicic was born on in Volosko, now in Croatia. After spending his early school years in Volosko, then Rijeka, he studied physics and mathematics at the Faculty of Philosophy, in Prague. In 1882, he started his nine-year carrier in the Nautical School in Bakar teaching meteorology , and beginning his scientific work. A few years later, in 1887, he founded a meteorological station in Bakar.

In 1891, Mohorovicic transferred to Zagreb, and in 1892, he became the head of the Meteorological Observatory in Zagreb. Here he studied and wrote mainly about clouds , rainstorms, thunderstorms, tornadoes, whirlwinds, hail, and winds, focusing interest on these meteorological phenomena and their scientific interpretation, as well as studying the climate of Zagreb. In 1892, he started astronomical observations of stars. In 1893, he established a network of stations for thunderstorm observations. Still in 1893, he received his doctor of philosophy degree at the Zagreb University. In 1899, he founded hail stations, and the same year he started a research project about harnessing wind energy.

In 1901, Mohorovicic became director of the meteorological service of Croatia and Slovenia. In his meteorological research, he was still interested in the detection of tornadoes and thunderstorm tracking. Mohorovicic's last contribution to meteorology was in 1901, when he published his paper on vertically decreasing atmospheric temperature . After the turn of the twentieth century, Mohorovicic's scientific interest focused exclusively on the problems of seismology .

Mohorovicic gradually extended the activities of the observatory to other fields of geophysics: seismology, geomagnetism and gravitation, although as chief of the observatory, Mohorovicic was still responsible for recording all the meteorological data for Croatia and Slovenia. The earthquake in 1909 in Croatia directed his interest towards the examination of seismic waves, and in 1910, Mohorovicic published his findings. His plot (arrival time versus epicenter distance to recording station) used the data from 29 stations within 1,491 mi (2,400 km) of the epicenter. He concluded that at around 31 mi (50 km), there must be an abrupt change in the material in the interior of the earth, because he observed an abrupt change in the velocity of the earthquake waves. Although this conclusion was not accepted immediately, a few years later, in 1915, other researchers confirmed it. This discontinuity region under all the continents and oceans is today called the Mohorovicic discontinuity, or in short, the Moho. Although others later refined the study of crust and upper mantle with the application of new methods, Andrija Mohorovicic was clearly a pioneer of this area .

Mohorovicic also published a paper in 1909 on the effect of earthquakes on buildings that described periods of oscillation, which was considered by his contemporaries to be ahead of the times not only in his own country, but worldwide. In 1910, he became an associate university professor. From 1893 to 1917, he taught subjects in the fields of geophysics and astronomy at the Faculty of Philosophy in Zagreb. In 1893, Mohorovicic first became a corresponding member, then in 1898, a full member of the Academy of Sciences and Arts in Zagreb. Although at the end of 1921 he retired, he worked actively until the late 1920s. He died in 1936, and he is buried in Zagreb.

Because of his extensive work studying epicenters, seismographs, and travel-time curves, much of our knowledge of how earthquakes occur, as well as the current models of the earth's structure can be traced back to the work of Andrija Mohorovicic. Among his other achievements, Mohorovicic also found a procedure for identifying the unique location of earthquake epicenters and formulated an analytical expression for the increase of elastic wave velocity with depth, which was later named Mohorovicic's law. Mohorovicic's thoughts and ideas were original, and he focused his interest in more than one area: the effects of earthquakes on buildings, harnessing the energy of the bora, models of the earth, locating earthquake epicenters, seismographs, and many other subjects also in meteorology. Andrija Mohorovicic was an outstanding scientist and researcher, and his scientific work in the field of seismology rightfully gave him world recognition, making him one of the founders of modern seismology.

See also Earth, interior structure

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Mohorovičić, Andrija

Mohorovičić, Andrija (1857–1936) A Croatian seismologist, Mohorovičić discovered a seismic discontinuity (since named after him, see MOHOROVIČIĆ DISCONTINUITY) at the crust–mantle boundary, when analysing the results of an earthquake in 1909. He found two sets of P-wave arrivals with different travel times, and thus was able to calculate the depth of the discontinuity.

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